Elongated, flexible guides are often used in medical procedures to gain access to specific internal sites within the body without major surgery. Guides are advanced through the body, for example, through peripheral blood vessels, the gastrointestinal tract, or the urinary tract. Guides, often referred to as guidewires, are commercially available and are currently used, among other fields, in cardiology, electrophysiology, gastroenterology, urology, and radiology.
Once positioned indwelling, the guidewire defines the path for the introduction of catheters and other medical instruments to a desired site; however, such instruments are generally less wieldy than the guidewire, have significantly more mass, and create a risk of kinking the guidewire as they are advanced over the guidewire.
Typical guidewire constructions include a central core wire made of stainless steel or other metal which provides stiffness to the guidewire, and have a distal or forward end portion of increased flexibility to better enable the clinician to maneuver the guidewire into the appropriate passageway. The more proximal portion of the guidewire provides the requisite stiffness to support and guide the medical instrument to the site accessed by the guidewire. Depending on the design, the guidewire may include a coil along the distal portion of the guidewire, or which surrounds the entire core wire. Also, in some designs, the core wire is movable within the coil to permit the clinician to selectively adjust the flexibility of the guidewire as the guidewire is being positioned and while a catheter or other instrument is being advanced thereover. In designs which include a coil, a distal weld is commonly made at the distal end of the coil to provide an atraumatic tip, and a safety wire welded to the tip extends proximally, within the coil, to better ensure that the tip does not separate from the guidewire during use.
In most designs, the dimension of the core wire essentially defines the stiffness of the guide wire along its length. For a given core wire material, the greater its cross-section, the greater the stiffness of the overall guidewire. The choice of core wire material affects the performance characteristics of the guidewire, as well as its cost. Core wires made of stainless steel are inexpensive, but are prone to kinking during advancement of catheters and other instruments. Core wires made of fiberglass composites are more resistant to kinking but they are more prone to abruptly snapping, and it is difficult to provide a taper to the distal end of the fiberglass core, to improve its flexibility, without splintering. See U.S. Pat. No. 5,251,640 of Osborne. Core wires made of shape memory alloys that have been processed to be superelastic at body temperature remain expensive. Further, superelastic guidewires do not readily take a set, for example, a J-shaped tip or a hockey-stick tip, but rather require further process steps to render the tip portion non-superelastic.
U.S. Pat. No. 4,925,445 of Sakamoto et al., discloses a guidewire construction which includes a superelastic core wire surrounded by a thick polymer jacket. The jacket design builds up the thickness of the guidewire to the desired dimension without the need for a superelastic core wire that is substantially the same diameter as the overall outer diameter of the guidewire. A guidewire where the core wire was substantially the same diameter as the overall diameter of the diameter of the guidewire would be insufficiently flexible for some intended applications. The polymer jacket is expensive to apply, and is typically added by an over extrusion process, by heat shrinking tubing over the core wire, or by hot sizing the jacket material. Another problem with jacketed guidewire designs is that the dimensionally significant jacket placed over the core wire may obscure the performance characteristics of the core wire itself. Therefore, there remains a need for a design to build up the outer diameter, while allowing appropriate flexibility of the product.
To promote ease of insertion, withdrawal, and positioning of the guidewire, it is generally desirable to provide guidewires with a hydrophilic, lubricious outer surface. There are difficulties associated with hydrophilic, lubricous coatings applied directly to a metal coil. Such coatings have generally only been applied to polymer jacketed guide wires. Typically, the hydrophilic lubricous coating is a hydrogel material, and as such, when it is hydrated, it absorbs a significant amount of water, swells, and loses adhesion to the steel.
A desirable improvement in the art would be a flexible, low cost, kink resistant, low friction guidewire having a formable tip.